The present invention generally relates to optical systems and, more particularly, to an improved imaging apparatus of a small size whose application is in the optical exposure system of an electrophotographic copying machine or a graphic data reading device of a data processing input/output equipment for instance.
Various imaging systems have heretofore been proposed to provide an optical exposure system of an electrophotographic copying machine. A first type of system holds an imaging lens or an optical system stationary and move a document or a platen and a photosensitive member in timed relation to each other (synchronous movement type). A second type of system keeps a document or a platen fixed in place and moves a first mirror and a photosensitive element at a common velocity while driving a second mirror at a velocity half the velocity of the first mirror (mirror scan type). A third type of system moves a lens box relative to a document or a platen and a photosensitive member each held stationary (lens box scan type). All of these known optical systems are advantageous in one aspect but disadvantageous in another. For instance, where an object to be copies is a book or like bulky document, the second or third type of system mentioned in which the document or the platen remains stationary is advantageous over the first type of system which moves it.
A demand recently made on copiers is, among others, for a reduction of its overall size. This is naturally reflected by a demand for a small sized optical exposing system installed in a copier. To meet such a demand, discussion is now under way in the industry concerned to replace the traditional lenses with an array of optic image transmitting fibers or an array of microlenses as an image forming element of the optical exposing system. An image transmitting optic fiber is a rod-like transparent element or rod lens of a diameter of about 1 mm and whose refractive index varies parabolically from its center radially to its perimeter. When provided with a suitable length, a single optic fiber can form an erected equisized actual image of an object without any assistance. To construct an optical exposure system for practical use, multiple such elements having the same length, refractive index and diameter are line up in a row or array. A microlens array on the other hand comprises a number of microlenses common in curvature, refractive index and diameter arranged to form an array. Being a kind of spherical lenses, the microlens array needs at least three microlenses or microlense arrays superposed on the same optical axis in order that it can focus an erected equisized actual image of an object. A major drawback inherent in the optic fiber array is that each of its element must be processed by ion exchange or the like to vary the refractive index and such elements must be assembled into an array, which makes the production uneasy and adds to the cost. This also holds true in the case of the microlens array because a number of microlenses must have their optical axes exactly aligned together and light interception is indispensable between neighboring microlenses.
Prior art techniques concerned with microlens arrays include one disclosed in Japanese Patent Publication 49-8893 in which three flat microlens arrays are superposed to project an erected equisized image of an object However, difficulty is experienced in aligning the optical axes of the three independent lenses on a common optical axis and the six convex surfaces in total build up considerable abberations. Another known technique resides in superposing two relatively thick telecentric microlens arrays to form an erected equisized image as disclosed in Japanese Patent Application 53-122426 layed open to public inspection. This telecentric system is neither acceptable due to great flaring attributable to the thick lenses and due to difficult production. It has additional drawbacks that each lens array in the form of an integral flat molding must be bodily wasted when even one of the lenses is found inferior and that all the lenses in an array cannot be provided with identical performance without difficulty.
The imaging elements discussed have characteristically short focal lengths which enable them to reduce the overall dimensions of copiers. For this very reason, they are not suited for use with an optical exposure system of the aforementioned mirror scan type or the lens box scan type that commonly needs a long focal length. That is, an effort to have a long focal length with such an imaging element deprives it of the merit in making the whole apparatus small-sized. A prism and lens assembly is known as another imaging element which sets up a relatively long focal length while reducing the dimensions of an apparatus in which it will be incorporated. The prism and lens assembly has a triangular prism behind a spherical lens and can cut the distance between the lens and prism very short even if the focal length of the lens :s made long. It is possible therefore to arrange multiple such prism and lens assemblies in an array and effectively utilize this array as an imaging element of an optical exposure system of the mirror scan type or of the lens box scan type.
Prism and lens arrays known in the present stage of development all need, however, time consuming assembling work of independent lens elements and prism elements. This accompanies troublesome production, assembly and adjustment as well as disproportionate time required which promote an increase in the cost.